Effects

There are 24 proper Effects in Yume Nikki. Each effect changes Madotsuki's appearance and some provide special abilities.
Gathering all 24 effects is considered to be the primary objective of the game, and only when all effects have been gathered can the player see the game's finale by leaving them all in The Nexus and waking up.

The effects are scattered throughout the dream world, and some are more easily found than others. Some are found by touching particular objects whereas others are received from interacting with characters.
While none of the effects are mandatory to progress through the game, many events and locations are unreachable without them. They can be collected in almost any order, except for the 'Poop hair' effect, which requires either the Yuki-onna or Umbrella effects.

Practical Uses: As a snowman, interacting with Mafurako will always teleport you back by her side on the return trip, meaning you don't have to search for her to teleport again. In the Blazing Corridor, standing next to the fire will cause Madotsuki to slowly melt and become immobile.

Action: (1) Causes the screen to change colour as if bathed in a neon light. The effect wears off when the effect is unequipped. Pressing the 1 button again causes the screen to cycle randomly between shades of red, blue, yellow and violet.

The chair in Madotsuki's dream room can be ridden when Madotsuki sits on it then pulls the chair away.
Instead of writing in her diary, sitting at the desk and pressing Z (facing the desk) will let Madotsuki scoot the chair around her bedroom. She can ride the chair to almost anywhere in the dream world (albeit very slowly), but will not be able to get off it until she returns it to her desk.

The only involuntary effect, Madotsuki has a random chance of waking up with a crick in her neck. She will constantly be looking left. As she does with no effect, pressing 1 will cause her to sit down. She is unable to write in her diary or play NASU. Going back to sleep and waking up again should solve the problem.

Madotsuki will not dream that she has a crick in her neck, and yet there is a matching FC world sprite.

A useful glitch that quadruples Madotsuki's walking speed. Although Madotsuki will appear as her normal sprite (except in the FC world), the glitch will be active. The bicycle is the most commonly used effect for this, as it allows for quadruple speed. The Speed Glitch is an oversight caused by allowing the player to use the menu in the chair.

Instructions

Sit in the chair in Madotsuki's dream room

Open the menu with X

Equip the Bicycle

Press down, then Z to exit the chair.

The glitch will now be active. The effect will disappear if you equip any effects. Madotsuki will sit if 1 is pressed, but the other action will happen as well (e.g. with the bike, the bell noise will play).

A related glitch that is slightly more complicated, but allows the effect to stack allowing for movement eight times faster than the walking speed, which is equal to that of the Candle WorldToriningen.

Instructions

Sit in the chair in Madotsuki's dream room

Open the menu with X

Equip the Bicycle

Press Z to ride the chair

Ride the chair into the Nexus

Press 5 to drop the bicycle effect, then pick it back up

Open the menu (You should hear the 'unable' sound effect), and equip the bicycle

Return to the bedroom. There will be no chair at the desk.

Go to the desk and sit down on the invisible chair, then press down and Z to get up.

Speculation: It's possible this was intended to be the appearance Madotsuki takes on in the White Desert, as there is no counterpart FC world sprite. If this is the case, then it's likely that it was dropped since it could cause confusion as to whether the White Desert was really white, or whether the effect was a result of a problem with the player's perception of it (as is the case with old monochrome photos).
Allowing Madotsuki to retain her hues makes it clear that the player's vision is normal and that the world is white.

Another idea is that this is an effect that you get from the White Desert that makes Madotsuki (And possibly the room you are in) monochrome.